In addition to a main observer tube (tube used by a main observer/main user of the microscope), a co-observer tube (assistant observer tube) and thus a tube that may be used by a second observer, is frequently provided in surgical microscopes. Such a system is described in German patent application DE 43 31 635 C2 of the applicant and shown in FIG. 4, for example. As shown in FIG. 4, two pairs of stereoscopic (observation) optical paths entering the surgical microscope from a side where the observed object is to be located firstly traverse commonly (together and in the same direction) an object lens 3* and then are guided by a mirror arrangement 4* selectively to a main observer tube 2* or a co-observer tube 1*. Main observer tube 2* and co-observer tube 1* respectively comprise a system bending the respective pair of optical paths 2.1*, 1.2* and a pair of oculars 2.1* and 1.1*. Two focusing lenses 5a* and 6a* that are traversed by the optical paths of one pair of optical paths in sequence are located in the co-observer tube 1*. The optical paths, respectively, are guided separately by the focusing lenses, such that each focusing lens is only traversed by one single optical path of the pair of optical paths. Via the objective lens 3* and the focusing lens 5a* arranged closer to the object (and the objective lens 3*) than the other focusing lens 6a*, an object arranged in the object plane is imaged into a first intermediate image 7* located between the focusing lenses 5a* and 6a* for each optical path of the at least one pair of optical paths. These first intermediate images 7* are imaged into second intermediate images, which are located in a region of the system bending the optical paths 1.2* of the co-observer tube 1*. These second intermediate images are imaged by the oculars 1.1* of the co-observer tube 1*, respectively, in enlarged scale to the retina of an observer (namely the co-observer). An adaptation of a working distance of the co-observer tube 1* independently of the main observer tube 2* is possible by shifting one of the two focusing lenses 5a*, 6a*. However, if no appropriate correcting measure is taken, shifting of one of the two focusing lenses 5a*, 6a* results in a lateral drift of the intermediate images in a plane were the intermediate images are located, which plane is located in between the two focusing lenses 5a* and 6a*, and thus to a tilting of the stereoscopic optical axes. When the plane in which the field of view of a co-observer is located, is located above the focal plane of the main observer, the intermediate images observed by the co-observer drift inwardly towards one another, which requires a convergent eye position of the co-observer. When the plane observed by the co-observer is located below the focal plane of the main observer, the intermediate images observed by the co-observer drift outwardly and thus away from each other, resulting in a divergent eye position of the co-observer.
German Utility Model DE 295 05 755 U1 proposes a solution to the above problem of a convergent respectively divergent eye position of the co-observer. According to this document, an optical system of variable back focal length is proposed. The optical system has a main objective of constant focal length and at least two groups of lenses that are arranged in sequence to the main objective in the stereoscopic partial optical paths and are shiftable along the optical axis for variable adaptation of the back focal length and focal length. Upon shifting of at least one of the two groups of lenses in the stereoscopic partial optical paths along the optical axis, a simultaneous displacement of the group of lenses in a direction orthogonal to its optical axis is performed. The two groups of lenses in the stereoscopic partial optical paths are each designed as achromats, respectively separately corrected with respect to aberrations.
It is a disadvantage of known stereoscopic microscopes that, upon focusing, either a drift of the intermediate images imaged in the optical paths occurs, resulting in an uncomfortable situation for a user, or a high effort is necessary regarding optical components, respectively, mechanical components, to avoid such a drift of the intermediate images.